The present invention relates broadly to packing rings and like seals for a reciprocating shaft such as the rod of a hydraulic or pneumatic cylinder or other actuator, and more particularly to a sealing system therefor which is engineered as having a check valving function to relieve fluid pressure developed between a primary and secondary seal member of the system.
Fluid seals, also known as packing rings, for machine part joints are well-known in the art. A typical application therefor involves the provision of a fluid seal intermediate relatively movable or reciprocating surfaces such as the outer surface of the rod of a hydraulic or pneumatic cylinder, or other fluid actuator, which reciprocates axially relative to an internal bore or other inner surface of a stationary, surrounding housing. Such seals, known in the vernacular as rod seals, conventionally are configured in a free state as a generally annular element which is molded or otherwise formed of an elastomeric or other resilient material such as a synthetic, natural, or co-polymer rubber, or a polymeric material such as a silicone, fluoropolymer, or, preferably, a polyurethane or fluoropolymer. Typically, the rod seal element is seated within an annular sealing gland which is provided within one of the surfaces, such as an internal bore of the cylinder housing, with the opening of the gland oriented as facing the other surface such as the outer surface of reciprocating rod. Within the gland, the seal is interposed between the rod, which is received coaxially through the seal for reciprocation within the cylinder bore, and a circumferential, peripheral side wall of the gland. As the piston rod reciprocates along a longitudinal axis within the bore, the seal functions to engage the reciprocating surface of the piston and the side wall of the gland to provide, respectively, a dynamic and static seal therebetween. In this regard, as installed under stress within the gland the seal presents in an energized or deformed state low and high pressure axial surfaces each disposed adjacent a corresponding side of the fluid pressure system, and inner and outer radial surfaces each defining one or more contact surfaces or "lips" with a corresponding, opposing surface of the rod and gland.
Ideally, a rod seal element should provide an effective sealing capability under static and dynamic conditions, at both low pressure, as the rod extends, and high pressures, as the rod retracts, with a minimum of static and dynamic friction for a long, maintenance-free service life. However, the seal must function not only to seal the rod against leakage, but also to wipe the rod of any liquid or other fluid film which may adhere to the outer surface thereof as the rod is extended outward of the packing gland and cylinder housing. Similarly, as the rod is retracted, it is desirable again wipe the rod to remove any contaminants therefrom which could damage the packing or which could be carried past the packing and into the cylinder. To provide these noted sealing and wiping functions, it is customary to separate wiping and sealing members in a series arrangement with the wiper mounted a spaced-apart axial distance downstream of the seal. In general configuration, the wiper member may include an elongate wiper arm for a large surface area contact with the outer surface of the rod. The seal member, in turn, may be configured as a U-cup having a low pressure side heel portion and a high pressure side axial surface which is asymmetrically bifurcated by a radially-extending channel to define, in a stressed orientation of the seal, an inner sealing lip for dynamic contact with the outer surface of the rod, and an outer sealing lip for static contact with the peripheral surface of the gland. Representative rod sealing and wiping elements are described in International Publication No. WO 92/15807 and in the following U.S. Pat. Nos.: 5,553,872; 5,482,296; 5,431,415; 5,143,382; 5,104,131; 4,935,876; 4,893,823; 4,889,349, 4,850,602; 4,723,782; 4,553,761; 4,449,718; 4,417,503; 4,337,956; 4,328,972; 4,268,045; 4,231,578; 4,174,846; 4,155,557; 3,942,806; 3,921,991; 3,790,1792,521,248; 3,339,932; 3,169,776; 2,997,318; 2,979,350; and 2,907,596. Commercial rod seals and wipers are manufactured, for example, by the following: Parker-Hannifin Corporation, Packing Division, Salt Lake City, Utah; Parker Hannifin GmbH, Pradifa Packing Division, Bietigheim-Bissingen, Fed. Rep. of Germany; Power-Seal Corporation; Freudenberg-NOK, Milan, Ohio; W.S. Shamban Co., Santa Monica, Calif.; Busak & Luyken GmbH & Co., Stuttgart, Fed, Rep. of Germany; Martin Merkel GmbH & Co. KG, Hamburg, Fed. Rep. of Germany; and Greene Tweed, Kulpsville, Pa.
An additional upstream element optionally may be provided as a buffer ring which is interposed between the U-cup and the high pressure side of the system. As the name implies, the buffer ring functions to protect the U-cup from shock loads, but also cooperates with the U-cup in sealing a majority of the fluid. Conventional buffer rings typically are formed as having a symmetrical, rectilinear cross-sectional profile.
It has been observed, however, that with buffer rings of a conventional configuration, problems can develop under severe operating conditions. For example, if the ring is too aggressive in sealing fluid, sufficient lubrication may not be provided to the rod. Excessive wear, increased frictional drag, and premature seal failure therefore can result. In contrast, if the buffer ring admits fluid only in one axial directional, that is, to the U-cup, during the extension stroke, fluid can become trapped between the buffer ring and the U-cup. During the retraction stroke of the rod, fluid pressure thereby may be developed between the ring and U-cup which can exceed the system pressure. This pressure which may exceed 400 bar, in turn, can cause both the U-cup and the ring to be extruded into the gap which is provided between the cylinder rod and housing. Of course, such extrusion represents an unacceptable condition in that subsequent rod stroke cycles can cause material to be abated from the U-cup or ring, resulting in the failure of the rod sealing system.
Recently, however, an improved buffer ring/U-cup arrangement has been introduced by Parker-Hannifin Corporation's Packing Division under the tradename "Parker BT/BR Profile Rod Sealing System." As is described by Swanson, R., in "OEM Off-Highway,"January 1995, the improved buffer ring profile includes an inside diameter which is formed as a solid, back-beveled sealing lip that provides dynamic sealing contact with the cylinder rod. As is in earlier designs, the solid sealing lip develops a sealing force against the rod and protects the fluid system from shock loads. The outside diameter, however, is uniquely formed to provide a dynamic, check valve portion for contact with the static side peripheral surface of the gland. When energized by the fluid pressure developed between the buffer ring and the U-cup, the check valve portion is actuable to relieve the pressure back into the fluid system. Advantageously, the fluid pressure is vented around the less critical outside or static side diameter of the ring such that a constant, dynamic sealing contact may be maintained on the rod by the more critical inside diameter of the ring. Moreover, as the outside check valve portion in the subject ring is not subject to the reciprocating motion of the rod, that portion may be made smaller and thinner than if provided on the inside diameter of the ring to afford greater design flexibility and selection in the check valving function.
Another buffer ring design is described in U.S. Pat. No. 4,553,761 as having a radially-outside, statically-acting sealing lip for bearing on the housing groove, and a radially-inside, dynamically-acting sealing lip for bearing on the axially-displaceable surface of a piston rod. The axial spacing of the statically-acting sealing lip from the adjacent end face of the groove is provided to be greater than that of the dynamically-acting sealing lip. As a result, the movement of the rod in the direction of the sealing lips tilts the profile of the ring such that the end of the seal opposite the sealing lips is lifted away from the surface of the moving rod. Such lifting provides a pressure release whereby leakage liquid which is entrained by the moving rod and which possibly is accumulated in the space between the ring and a downstream seal is vent between the inside diameter of the ring and the rod back into the fluid system.
U.S. Pat. Nos. 4,723,782 and 4,953,876 disclose another arrangement for sealing between an axially-movable rod of a hydraulic device and a housing wall through which the rod moves. The arrangement includes an inner sealing ring and an outer stressing ring which are accommodated jointly within a groove of the housing wall such that the stressing ring contacts the sealing ring along a common contact face, with the sealing ring, in turn, contacting the rod under the force of the stressing ring. The sealing ring is provided on its inner surface which faces the rod with a pair of axially-offset sealing edges that define an annular space therebetween. Such space is connected with the outer surface of the sealing ring by an internal channel which extends from the inside diameter of the sealing ring to the outside diameter thereof to open at a point covered by the stressing ring which is located near the high-pressure end of the common contact face of the sealing and stressing rings. The channel and the stressing ring are stated to form a non-return valve which allows any fluid pressure that has entered the annular space to be relieved through the channel against the surface pressure developed on the outside diameter opening of the channel by the stressing ring. In this way, the pressure in the annular space may be controlled to limit leakage resulting from the outer sealing edge of the sealing ring being lifted from the surface of the rod.
U.S. Pat. No. 5,431,415 discloses a seal element for installation in an annular groove of a housing for a reciprocating rod. The seal element has a first radial surface that forms an acute heel with a first end surface of the groove. Upon the application of fluid pressure, the seal element is compressed into the groove such that the first radial surface of the element is moved into contact with the first end surface of the groove to prevent contamination from entering the static area. Angled surfaces on the inner and outer diameters of the element define high and low pressure sealing lines or lips for contact with a corresponding axial surface of the groove or rod. When low pressure conditions are applied to the element during the extension of the rod, the element rotates to close the heel angle providing an inner static area between the first radial surface of the seal element and the first end surface of the groove. When high pressure conditions are applied to the element during the retraction of the rod, the first radial surface of the element is maintained in continuous sealing engagement with the corresponding end surface of the groove. Contaminants thereby are excluded from the sealing groove during both high pressure and low pressure operation to maintain the integrity of the static seal developed between the seal element and the groove.
U.S. Pat. No. 4,337,956 discloses a sealing device for installation in a groove formed within one of an opposing pair of generally parallel surfaces. The sealing device includes a base member having bottom surface contours which define a pair of spaced-apart, longitudinally extending fluid passages between the base member and the groove. A pair of lip members are connected to the base member to extend therefrom for engagement with the other one of the confronting surfaces to thereby form another fluid passageway between the lip members and that surface. An internal channel is formed through the device to provide fluid communication between the base and lip member passages.
The demands placed on fluid power equipment, such as hydraulic cylinders and other actuators, by higher operating pressures, harsher service environments, and other conditions continue to increase. It therefore will be appreciated that further improvements in rod seals and sealing systems therefor would be well-received by the industry. Preferred improvements would enhance sealing performance by limiting fluid leakage and thereby reducing the potential for the introduction of contaminants into the fluid system.